"The reason of this is quite obvious in some cases; the female hybrids, although possessing all the external appearances and characteristics of perfect animals, are physiologically imperfect and deficient in the structural parts of the reproductive elements necessary to generation. It is said to be invariably the case with the male mule, the cross between the ass and the mare; and hence it is that although crossing the horse with the ass is easy enough, and is constantly done as far as I am aware, if you take two mules, a male and a female, and endeavor to breed from them, you get no offspring whatever; no generation will take place. This is what is called the sterility of the hybrids between two distinct species." (Huxley, _"On the Origin of Species."_ p. 212.) He continues: "Thus you see that there is a great difference between 'mongrels,' which are crosses between distinct races, and 'hybrids,' which are crosses between distinct species. The mongrels are, so far as we know, fertile with one another. But between species, in many cases, you cannot succeed in obtaining even the first cross; at any rate it is quite certain that the hybrids are often absolutely infertile one with another. "Here is a feature, then, great or small as it may be, which distinguishes natural species of animals. Can we find any approximation to this in the different races known to be produced by selective breeding from a common stock? Up to the present time the answer to that question is absolutely a negative one. As far as we know at present, there is nothing approximating to this check. In crossing the breeds, between the fantail and the pouter, the carrier and the tumbler, or any other variety or race you may name--so far as we know at present--there is no difficulty in breeding together the mongrels." However, he continues, as soon as you remove the conditions which produced the new variety,--as when you permit pigeons to mate promiscuously,--no matter how different the varieties may have been, you will have, in a few generations of pigeons, the same blue rock pigeon with the black bars across the wings. No new species has originated. All varieties, in a free state, revert to type. "This," says Huxley, "is certainly a very remarkable circumstance."